The present invention relates to a digital video tape recorder (hereinafter referred to as digital VTR) having a track format for recording digital video and audio signals in predetermined areas on oblique tracks, and relates to a digital VTR in which the digital video and audio signals are input in the form of a bit stream, and the bit stream is magnetically recorded and played back.
FIG. 93 is a diagram showing a track pattern of a conventional, general consumer digital VTR. Referring to the drawing, a plurality of tracks are formed on a magnetic tape 10, in a head scanning direction inclined to the tape transport direction, and digital video and audio signals are recorded therein. Each track is divided into two areas, a video area 12 for recording a digital video signal and an audio area 14 for recording a digital audio signal.
Two methods are available for recording video and audio signals on a video tape for such a consumer digital VTR. In one of the methods, analog video and audio signals are input, and recorded, using a video and audio high-efficiency encoding means; this is called a baseband recording method. In the other method, the bit stream having been digitally transmitted; this method is called a transparent recording method.
For the system of recording ATV (advanced television) signals, now under consideration in the United States, the latter transparent recording method is suitable. This is because the ATV signal is digitally compressed signals, and does not require a high-efficiency encoding means or a decoding means, and because there is not degradation in the picture quality due to transmission.
The transparent recording system however is associated with a picture quality problem in a special playback mode, such as a fast playback mode, a still mode and a slow mode. In particular, when a rotary head scans the tape obliquely to record a bit stream, almost no image is played back at the time of fast playback, if no specific measure is taken.
An improvement for the picture quality for the transparent recording system recording the ATV signal is described in an article Yanagihara, et al, "A Recording Method of ATV data on a Consumer Digital VCR", in International Workshop on HDTV, 93, Oct. 26 to 28, 1993, Ottawa, Canada, Proceedings, Vol. II. This proposal is now explained.
In one basic specification of a prototype consumer digital VTR, in SD (standard definition) mode, when the recording rate of the digital video signal is 25 Mbps, and the field frequency is 60 Hz, two rotary heads are used for recording a digital video signal of one frame, being divided into video areas on 10 tracks. If the data rate of the ATV signal is 17 to 18 Mbps, transparent recording of the ATV signal is possible with the recording rate in this SD mode.
FIG. 94A and FIG. 94B show tracks formed in a magnetic tape using a conventional digital VTR. FIG. 94A is a diagram shoving scanning traces of the rotary heads during normal playback. FIG. 94B shows scanning traces of the rotary heads during fast playback. In the example under consideration, the rotary heads are opposite each other spaced 180.degree. apart on a rotary drum, and the magnetic tape is wrapped around over 180.degree.. In the drawing, adjacent tracks on the tape 10 are scanned by two rotary heads A and B having different azimuth angles, alternately and obliquely, to record digital data. In normal playback, the transport speed of the tape 10 is identical to that during recording, so that the heads trace along the recorded tracks. During fast playback, the tape speed is different, so that the heads A and B trace the magnetic tape 10 crossing several tracks. The arrow in FIG. 94B indicates a scanning trace by head A at the time of five-time fast feeding. The width of arrow represents the width of the region read by the head. Fractions of digital data recorded on tracks having an identical azimuth angle are played back from regions meshed in the drawings, within five tracks on the magnetic tape 10.
The bit stream of the ATV signal is according to the MPEG2 standard. In this bit stream according to the MPEG2 standard, only the intra-frame or intra-field encoded data of the video signal, i.e., the data of intra encoded block (intra encoded block) alone can be decoded independently, without reference to data of another frame or field. Where the bit stream is recorded in turn on the respective tracks, the recorded data are replayed intermittently from the tracks during fast replay, and the image must be reconstructed from only the intra-encoded blocks contained in the replay data. Accordingly, the video area updated on the screen is not continuous, and only the fractions of data of intra coded block are replayed, and may be scattered over the screen. The bit stream is variable-length encoded, so that it is not ensured that all the replay data over the screen is periodically updated, and the replay data of certain parts of the video area may not be updated for a long time. As a result, this type of bit stream recording system does not provide a sufficient picture quality during fast replay in order to be accepted as a recording method for a consumer digital VTR.
FIG. 95 is a block configuration diagram showing an example of recording system in a conventional digital VTR. Referring to the drawing, reference numeral 16 denotes an input terminal for the bit stream, 18 denotes an output terminal for the bit stream, 20 denotes an HP data format circuit, 22 denotes a variable-length decoder, 24 denotes a counter, 26 denotes data extractor, and 28 denotes an EOB (end of block) appending circuit.
To improve the quality of fast replay pictures, the video area on each track is divided into two types of areas. That is, the video area on each track is divided into main areas 30 for recording the bit stream of the ATV signal, and copy areas for recording important part of the bit stream which are used for reconstruction of the image in fast replay. Only the intra-encoded blocks are effective during fast replay, so that they are recorded in the copy areas. To reduce the data further, only the low-frequency components are extracted from all the intra-encoded blocks, and recorded as HP (high priority) data.
The bit stream of MPEG2 is input via the input terminal 16, and output via the output terminal 18, without modification, and sequentially recorded in the main areas 30 on each track of the tape. The bit stream from the input terminal 16 is also input to the variable-length decoder 22, and the syntax of the bit stream of the MPEG2 is analyzed, and the intra-picture data is detected, and timing signals are generated by the counter 24, and the low-frequency components of all the blocks in the intra-picture data are extracted at the data extractor 26. Furthermore, EOBs are appended at the EOB appending circuit 28, and HP data is constructed at an HP data format circuit, not shown. The HP data is incorporated in the recording data for one track,and recorded in the copy areas 32.
FIG. 96A and FIG. 96B show an example of replay system in a conventional digital VTR. FIG. 96A schematically shows normal replay. FIG. 96B schematically shows fast replay.
Separation of data from the magnetic tape during normal replay and fast replay are performed respectively in the following ways. During normal replay, all the bit stream recorded in the main areas 30 is replayed, and the bit stream from the data separation means 34 is sent as the normal replay data, to an MPEG2 decoder, provided outside the replay system. The HP data from the copy areas 32 are discarded. During fast replay, only the HP data from the copy areas 32 are collected, and sent, as fast replay data, to the decoder. At the data separation means 34, the bit stream from the main areas 30 is discarded.
A method of fast replay from a track in which main areas 30 and copy areas 32 is next described. FIG. 97A shows a scanning trace of a head. FIG. 97B shows track regions from which the replay is possible. When the tape speed is an integer multiple of the normal playback speed, if phase-locking control is conducted by an ATF (automatic track following) method or the like for tracking by moving the head itself, the head scanning is in a predetermined phase relationship with tracks having an identical azimuth. As a result, the data replayed by the head A from the tracks recorded alternately by the heads A and B, are fixed to those from the meshed regions.
In FIG. 97B, if the signal having an output level larger than -6 dB is replayed by the heads, the data is replayed by one head from the meshed tape regions. The drawing shows an example of nine-time speed replay. If replay of the signals from the meshed regions is ensured at the nine-time replay, the regions are used as copy areas, and the HP data are recorded in the copy areas, so that the readings of the HP data from these regions at this speed is possible. However, reading of these signals at different speeds is not ensured. Accordingly, a plurality of areas need to be selected for the copy areas, so that the replay signals can be read at different tape speeds.
FIG. 98 shows regions where the copy areas overlap for a plurality of different replay speeds. It shows examples of scan regions for three different tape speeds, for cases where the head is in synchronism with an identical-azimuth track. The scan regions where the reading by the head is possible at different tape speeds overlap, at some of the regions. By selecting the regions at which the overlapping occurs as the copy areas, reading of the HP data at different tape speeds can be ensured. The drawings show the regions at which overlapping occurs at the fast-forward at four-time, nine-time, and 17-time speed. Theses scan regions are identical to those of feed-forward at -2-time, -7-time and -15-time high speeds (i.e., rewind at 2-time, 7-time and 15-time speeds).
Even though there are overlapping regions for different tape speeds, it is not possible to determine a recording pattern so that identical regions are always traced at different speeds. This is because the number of tracks crossed by the head differs depending on the tape speed. Moreover, it is necessary for the head to be capable of starting tracing at whichever identical-azimuth track. For this reason, identical HP data is repeatedly recorded over a plurality of tracks, to solve the above problem.
FIG. 99 shows examples of scanning traces of the rotary head at different tape speeds. Regions 1, 2 and 3 are selected from among the overlapping regions for five-time and nine-time speeds. If identical HP data are repeatedly recorded over 9 tracks, the HP data can be read at either of five-time and nine-time speeds.
FIG. 100A and FIG. 100B show scanning traces at five-time speed replay. In the illustrated example, identical HP data is repeatedly recorded over five consecutive tracks. As will be seen from the drawings, identical HP data is recorded over the number of tracks identical to the number of times of the tape speed (i.e., 5). In either of case 1 and case 2, either the head A or B can read HP data from corresponding azimuth track. Accordingly, providing the copy areas in each track, in a number identical to the number of times of the tape speed at the fast replay, and repeatedly recording the HP data there, the copied HP data can be read at various speeds, and in either the forward or reverse direction.
In the manner described, the special replay data is recorded in the copy areas, repeatedly, to improve the picture quality during the special replay in the transparent recording system.
FIG. 101 shows a recording format on a track in a conventional digital VTR. Main areas and copy areas are provided in one track. In a consumer digital VTR, a video area in each track has 135 sync blocks (SB), and 97 sync blocks are assigned to main areas and 32 sync blocks are assigned to copy areas. The sync blocks at the regions corresponding to the 4-, 9- and 17-time speed shown in FIG. 98 are selected for the copy areas. The data rate of the main areas is about 17.46 Mbps (97.times.75.times.8.times.10.times.30), and the data rate of the copy areas where identical data is repeated 17 times is about 338.8 kbps (32.times.75.times.8.times.10.times.30/17).
FIG. 102A and FIG. 102B show an example of the configuration of a track containing video and audio data.
The magnetic tape of a digital VTR according to the specification (hereinafter referred to as SD specification) defined by the SD mode, a video area of 149 SB and an audio area of 14 SB are provided on both sides of a gap, as shown in FIG. 93. and the video and audio data are recorded in these areas, together with error correction codes. Employed as the error correction codes for the video areas in the SD specification are (85, 77, 9) code (hereinafter referred to as C1 check code) in the recording direction (right-left direction in the drawing), and (149, 138, 12) Reed-Solomon code (hereinafter referred to as C2 check code) in the vertical direction. Employed as the error correction codes for the audio areas are (85, 77, 9) Reed-Solomon code (C1 check code) in the recording direction, like the video signal, and (14, 9, 6) Reed-Solomon code (hereinafter referred to as C3 check) in the vertical direction. Auxiliary data (VAUX data) is recorded in front of and at the back of the video data.
FIG. 103 shows an example of configuration of one sync block on the magnetic tape. As illustrated, the region of 1 SB is formed of 90 bytes, and a header consisting of sync pattern recording region 36 of two bytes, and ID signal region 38 of three bytes are formed at the head end, and recording region 42 for the error correction code (C1 check code, in the example illustrated) of 8 bytes is provided at the back of the data region 40 of 77 bytes. In FIG. 102A and FIG. 102B, the header parts are omitted.
Because the conventional VTR is configured as described above, and special replay data is repeatedly recorded in the copy areas, the recording rate for the special replay data is very low. In particular, the quality of the reconstructed pictures formed during slow replay or fast replay is low.
For instance, if the intra-frame is formed twice a second, the amount of data of intra-encoded blocks of the ATV signal is predicted to be about 3 Mbps. In the prior art, only 340 kbps can be recorded, and the quality of the reproduced picture is very degraded.
Moreover, the data for the respective fast replay speeds is recorded, being dispersed over a wide region. Accordingly, if the track is non-linear, it is difficult to achieve accurate tracking control over the entire data region, and the replay signal from some of the regions may not be of a sufficient level.
Furthermore, during special replay (fast replay, slow replay, still replay and the like), the rotary head crosses a plurality of recording tracks obliquely to pick up the replay data intermittently, as was described above. It is therefore not possible to form error correction block (video data) shown in FIG. 102A and FIG. 102B from the replay data during special replay. That is, during special replay, the error correction using C2 or C3 check code is not performed, but error correction using C1 check code alone is applied to the replay data.
If the error correction using the C1 check code alone is applied, if the symbol error rate 0.01, the error detection probability is 1.56.times.10.sup.-3. This means one error per about 8 sync blocks is detected. Because the replay data output is not stable during special replay, so that the symbol error rate can often be more than 0.01. Moreover, the recording data is variable-length encoded, so that when an error is present, the succeeding replay data cannot be used, leading to degradation in the picture quality. The rate of undetected errors is also about 7.00.times.10.sup.-8. Thus, the frequency of occurrence of undetected errors is high.
Moreover, during fast replay, the data rate is low, and only the low-frequency components are replayed, so that the resolution of the picture is poor.
Furthermore, it is necessary to pick up data for a plurality of fast replay regions in one scanning of the head during fast replay, so that when the track is no-linear, or when the scanning trace is non-linear, the data at the fast replay region where the non-linearity is present cannot be reproduced.
Moreover, since it is necessary to pick up data for a plurality of fast replay regions by one scanning of the head, replay can be performed only at certain speeds. The speed at which replay can be performed is limited, and the number of the replay speeds is small.
Moreover,the rotary speed of the drum of the four-head configuration is half that of the drum of two-heal configuration, so that the angle with which the head scanning trace crosses the track is larger, and the replay with the four-head configuration drum from the fast replay region is possible only at a speed half the speed at which the replay with two-head configuration drum is possible from the same fast replay region.
Furthermore, when the level of the replay signal fluctuates, the sync bit and the succeeding ID bits, and the first parity are reproducible, and the succeeding digital data is reproducible only up to its middle, and the rest cannot be reproduced because of the decrease in the level of the replay signal. In such a situation, the errors in the digital data is not detected until the result of the check using the second parity is produced. It is therefore necessary to conduct the predefined calculation for performing the check, and time is spent before the error detection.
Moreover, the amplitude of the replay signal varies periodically because the head crosses the recording tracks, so that burst errors frequently occur, and this cannot be detected easily nor quickly.
Moreover, the data used for fast replay is formed by extracting part of the data of the packets transparent-recorded, so that the length of data for forming a block of image is shorted. For this reason, when recording is made for the region used for transparent recording, disposing sync, ID, header, and packets in a predefined format, the fast replay signal cannot be recorded using the same format. The recording signal format forming means is therefore complicated.
Moreover, the fast replay data is used in common for all the replay speeds, so that the period at which one screen of image data is reproduced and displayed during fast replay at each speed is determined by the time for which the region in the tape longitudinal direction in which one screen for fast replay is recorded. Accordingly, the time for which one screen of image data is reproduced is inversely proportional to the speed. With higher speed, the picture changes quickly, while with lower speed, the picture changes slowly. As a result, the displayed image is easy to see for the viewer.
Furthermore, the region used for recording fast replay signal is limited to the region where reproduction is possible commonly for a plurality of fast replay speeds. Accordingly, the number of sync blocks for recording the fast replay signal is limited to the head scanning traces at the time of highest-speed replay, and the amount of data which can be recorded is small.
Moreover, when considering the fluctuation in the position of the head scanning trace due to fluctuation in the tape transport speed or the drum rotary speed, the region from which the data is reproduced without fail during fast replay is further reduced. This is particularly problematical in connection with fast replay with a higher speed.